In the case where a hydraulic system with which a work machine such as a hydraulic excavator is equipped operates for a long period of time, a reduction in performance of, especially, a hydraulic pump of main components of the hydraulic system due to wear and tear cannot be avoided. For this reason, the hydraulic pump is typically replaced periodically to maintain the performance of the hydraulic system and to avoid a risk of the occurrence of, for example, a malfunction and wear and tear of the hydraulic pump as much as possible.
In some cases, however, such a preventive measure increases cost when the degree of wear and tear of the hydraulic pump is excessively estimated and the period of a replacement cycle is too short, or cannot sufficiently reduce the frequency of the reduction in performance and the malfunction of the hydraulic pump, which affect works, when the period of the replacement cycle is too long. For this reason, it is necessary to know the state of the hydraulic pump with high precision during normal operation of the work machine. In a conventional technique against this problem, the amount of drain of the hydraulic pump is measured to estimate the degree of wear and tear of the hydraulic pump by using a property of a drain flow rate that increases with an increase in gaps between the components of the hydraulic pump due to wear.
A pump monitoring device of a hydraulic work machine that supplies hydraulic oil that is discharged from variable displacement hydraulic pumps and that merges through a check valve to an actuator to drive a work member is known as another conventional technique, and the pump monitoring device monitors whether any one of the variable displacement hydraulic pumps malfunctions on the basis of the total flow rate of the hydraulic oil discharged from the variable displacement hydraulic pumps (see, for example, PTL 1).